The authors retrospectively evaluated the relation between birth weight and prostate cancer (1986-1994) among 21,140 men of the Health Professionals Follow-up Study who reported in 1994 their weight at birth. No relation between birth weight and prostate cancer (n=545) was observed in multivariate logistic models. For high stage/grade tumors (n=213), compared with birth weights <7.0 lbs (<3,175 g), the relative risks were 1.20 (95% confidence interval (CI) 0.79-1.83) for 8.5-9.9 lbs (3,855.6-4,490.6 g) and 1.30 (95% CI 0.80-2.10) for > or = 10 lbs (> or = 4,536 g). These findings do not support an overall association between birth weight and prostate cancer incidence, but the possibility of a modest positive association between birth weight and high stage/grade prostatic cancer cannot be excluded.